The field of this invention is in the preparation of calcium phosphate minerals for bone cement or bone filler applications. More specifically, this invention relates to a calcium phosphate bone cement which consists of a mixture of tetra-calcium phosphate and di-calcium phosphate in an aqueous mixture, which mixture then sets to form a bone cement with a substantial portion of the cement being hydroxyapatite.
Hydroxyapatite is the major natural building block of bone and teeth. It has been found useful in fixing fractures and bone defects to use bone cements which are formed by combining calcium and phosphate precursors in an aqueous solution which initially forms a paste but then hardens into a hydroxyapatite bone cement. Hydroxyapatite has a calcium to phosphorous ratio of approximately 1.67 which is generally the same as the calcium phosphate ratio in natural bone structures. The paste may be placed in situ prior to setting in situations where bone has been broken, destroyed, degraded, become too brittle or has been the subject of other deteriorating effects. Numerous calcium phosphate bone cements have been proposed such as those taught by Brown and Chow in U.S. Reissue Pat. Nos. 33,161 and 33,221 and by Constantz in U.S. Pat. Nos. 4,880,610 and 5,047,031, the teachings of these patents are incorporated herein by reference.
It has been well known since the initial use of calcium phosphate cements that the addition of sodium phosphate solutions, potassium phosphate solutions or sodium carbonate solutions to the aqueous setting solution of the calcium phosphate precursors can speed setting times. This is documented in the Chow et al. April, 1991 IADR Abstract No.: 2410 and AADR, 1992 Abstract No.: 666 and was well known to those skilled in the art prior to these publications. In addition, such is discussed in Constantz et al. U.S. Pat. Nos. 5,336,264, 5,820,632 and 6,002,605.
Controlling bone cement or filler setting time is important because it is necessary for the material to be initially flowable and then moldable but it also must be able to set in place even though quantities of blood may be present. This blood creates problems in providing too much fluid so that the mineral salts are either washed away or dissolved prior to setting up. Obviously, if the material sets up too quickly, the surgeon has difficulty in mixing the calcium phosphate mineral cement and then placing it at the necessary bone site.
It has been found that the addition of sodium phosphate to the aqueous solutions mixed with the dry calcium phosphate precursors speeds the setting of calcium phosphate bone cement, such as those disclosed by Brown and Chow. However, the sodium phosphate may cause the setting to occur more quickly than desired. Thus, a method for better controlling the setting time of such cements has been found desirable.